This invention relates in general to devices for collecting solar energy and putting the collected energy to use and, more specifically, to a high efficiency solar panel for producing high pressure, super heated steam in a boiler heated by collected solar energy.
A great many different devices have been developed for collecting energy from the sun and using the collected energy to perform useful work. These vary from very simple, low efficiency passive devices to very complex devices for special purposes. Some merely collect heat energy in a liquid and direct it to a location which is to be heated. Photovoltaic cells convert the sun's energy directly to direct current electricity, generally with rather low energy conversion efficiency.
Simple passive devices using dark-colored tubes exposed to the sun through which water is flowed are often useful in heating swimming pools and the like. Typical of these passive systems is that described by Moseley in U.S. Pat. No. 2,608,968. These systems, although inexpensive, have low energy conversion efficiency and are not useful for purposes requiring high energy density, such as the generation of steam to, for example, drive electrical generators.
Other systems use very large arrays of reflectors to reflect solar energy to a small central energy absorber and boiler, such as is shown by Clark in U.S. Pat. No. 4,196,289. These systems require a large number of reflectors, each having a mechanism to keep the reflector at the proper angle to the sun and the central absorber. These tend to be very complex and expensive and require large areas for the assembly. Typically, these systems have a large number of reflectors arrayed across the ground, reflecting energy to a boiler mounted at the top of a tower at the center of the reflector field.
In order to improve efficiency, some solar energy systems use lenses or parabolic reflectors to concentrate energy received over a large area to a small absorber, typically a tube carrying a liquid such as water. Typical of these are the complex lens systems of Doundoulakis (U.S. Pat. No. 4,307,711) which focus energy from a large area onto a series of points along a tube. The complex lens must be very precisely made and carefully adjusted to accomplish reasonably high accuracy. Weslow in U.S. Pat. 4,137,899 attempts to accomplish similar concentration using a very large number of condensing lens embedded in a plastic sheet. A cylindrical lens or linear parabolic reflector is proposed by Holt in U.S. Pat. No. 4,058,110 to concentrate solar energy onto a line along a tube. These systems when exposed to the atmosphere quickly become degraded by dust or rain residue and require frequent cleaning due to the trough-like shapes used.
While many solar energy collectors are fixed in one position, such as on the surface of a south facing house roof, others are adjustable to allow orientation toward the sun and some have mechanically or electronically controlled tracking systems to follow the sun over the day. Typical of these is the system disclosed by Hammons in U.S. Pat. No. 4,225,781 which uses a plurality of photocells on and around a shade horn support so that as the position of the sun varies, different photo cells will be shadowed and the resulting change in cell output can direct a motor to move the collector appropriately. Others, such as Smith in U.S. Pat. No. 4,179,612 use dividers between a plurality of photo cells so that the dividers shadow certain cells as the relative position of the sun changes. Moving means moves the collectors in accordance with the varying photo cell output. While somewhat effective, these systems are not capable of consistently pointing the collector within about 1/2.degree. (the diameter of the sun as viewed from the earth) and so produce lower energy collection efficiency.
Thus, there is a continuing need for improvements in devices for the collection and utilization of solar energy. Improvements in system accuracy and reliability are needed, as are improvements in the collection and transfer of energy to improve the percentage of the incident energy that reaches the final work producing output device, such as a steam driven electric generator. Also, the efficiency of the output device in producing maximum work in proportion to the received energy needs improvement.